1. Field of the Invention
The invention relates to a lighting device for a display, and in particular, to a lighting device used for a display in which a light source constituting the lighting device is supplied with power from a power supply line.
2. Description of the Related Art
In recent years, a display called FPD (Flat Panel Display), which is thin and flat, has been developing a larger market comparing with a display in which a CRT (Cathode Ray Tube) is used. Among various types of FPDs, plasma displays and liquid crystal displays are the mainstream. Both have characteristics, respectively. A plasma display utilizes a discharge phenomenon so as to enable a large screen and high luminance. On the other hand, a liquid crystal display can cope with various sizes, and enables a compact size, low power consumption, light weight and high contrast.
A liquid crystal display enabling low power consumption, a thin type, and light weight, and capable of coping with various sizes, as described above, has such a configuration as shown in FIG. 54. That is, on the inner faces of two glass substrates 400A and 400B disposed opposite each other, transparent electrodes 401A and 401B are arranged in grids and orientation films 402A and 402B are arranged, respectively. In between the orientation films 402A and 402B, there is filled a liquid crystal material 403 having a characteristic of changing the orientation of its molecules upon being applied with an electric field so as to change the polarizing characteristics of a light passing, and on the outer sides of the two glass substrates 400A and 400B, polarizing plates 404A and 404B are adhered.
Further, in a color liquid crystal display, a color filter 405 with colors such as R, G, and B is arranged in between the glass substrate 400B on the display side opposite the light disposing side and the transparent electrode 401B. In the liquid crystal display as described above, the liquid crystal material 403 is electrically controlled by a TFT so as to control the polarizing characteristics of a light passing through each element, and to control the passing amount of the light with the polarizing plates. In other words, in the liquid crystal display, a voltage is applied to the liquid crystal material so as to control the polarizing characteristics of light passing therethrough to thereby display images.
As described above, a liquid crystal display is not a self-emitting element which emits light by itself but a passive element which displays images with light from the outside. Lighting devices are widely used for the passive elements as frontlights or backlights.
The invention relates to a lighting device for lighting an apparatus having a display means capable of displaying images with light from the outside as described above, and such a lighting device is referred to as a lighting device for a display herein.
The lighting devices of liquid crystal displays mainly include two types. That is, a vertical (direct) type enabling high luminance, high efficiency and enlargement in size, and an edge light (side light) type enabling reduction in size, reduction in thickness and low power consumption. In the vertical type, lamps are arranged on the rear face of the liquid crystal panel and direct light is used to thereby form flat light sources with high luminance and high efficiency. However, the vertical type has such demerits as a difficulty in reducing thickness and a large power consumption.
On the other hand, in the edge light type, one or more light sources are arranged on a side face of a plate, made of acrylic, called a light guiding plate which diffuses and scatters light to form flat light sources, to thereby realize a reduction in thickness and low power consumption. However, the edge light type has such demerits that the size of the light guiding plate becomes larger as the screen size becomes larger, and the weight becomes heavier. Therefore, the edge light type is adopted in many mobile devices which are small and in which power consumption is a great concern.
The edge light type is further divided into two types, that is, a backlight type in which a lighting device is arranged on the rear face of the liquid crystal display, and a frontlight type in which a lighting device is arranged on the front face so as to utilize reflection light by a reflective or a semi-transmitting (hybrid of reflection and transmission) liquid crystal display.
The service life of a liquid crystal display depends mostly on the service life of the light source. By performing maintenance to replace failed light source to a new one, there is an advantage that a liquid crystal display can be used semi-permanently.
FIG. 55 shows a structure to supply power to a linear light source of an edge light (side light) type backlight. Electrode terminals 2A and 2B protruded from both ends of a linear light source 1 are connected by soldering to core lines of power supply lines 3 and 4 connected to an inverter. The power supply line 3 connected by soldering to the electrode terminal 2B on the GND side (low pressure side) is interconnected so as to return to the HOT side (high pressure side) through the back face side of a reflector 5 surrounding the linear light source 1. Note that a vertical (direct) type backlight also has a similar structure to supply power to a linear light source.
In the aforementioned structure, however, the power supply lines 3 and 4 and the electrode terminals 2A and 2B are connected by soldering, and the power supply line 3 on the GND side is interconnected bypassing the rear face of the reflector 5. Therefore, in a case of maintenance when the linear light source 1 is failed, it is difficult to remove only the linear light source 1, causing a problem of poor performance in maintenance and recycle. In particular, when a lamp, containing mercury which is a harmful substance, is used as a linear light source, it is preferable to solely remove the lamp safely so as to change it to a new one easily. Further, in manufacturing, there is required an operation of soldering the power supply lines 3 and 4 to the electrode terminals 2A and 2B of the linear light source 1, respectively, causing a problem of increasing the number of operations and operational cost.
Considering the aforementioned problems, Japanese Patent Application Laid-open No. 2002-260796 proposes a contact member 6 for connecting a fluorescence tube 9 for backlight and a power supply line 12 without soldering, as shown in FIG. 56.
The contact member 6 has a first hole 15 and a second hole 16 at both ends of a base body 7 made of conductive resin, the outer faces of the base body 7 being covered with a covering member 8. The fluorescence tube 9 for backlight is so configured that an electrode terminal 10 at the tip thereof is outwardly fitted with a conductive member 11, and the power supply line 12 is so configured that a lead wire 13 at the tip thereof is covered with a conductive member 14. The conductive member 11 at the tip of the fluorescence tube 9 for backlight is inserted in the first hole 15, and the conductive member 14 at the tip of the power supply line 12 is inserted in the second hole 16, so that power is supplied to the fluorescence tube 9 for backlight via the base body 7 of the contact member 6.
However, according to the contact member 6 described in the aforementioned publication, it has conductivity with conductive resin, whereby it is less conductive comparing with a metal, causing an energy loss. Further, since the contact member 6 is so configured that the conductive members 11 and 14 of the fluorescence tube 9 for backlight and of the power supply line 12 are inserted in the first hole 15 and the second hole 16, respectively, a problem of low reliability in electrical connection is caused. Further, since it is required to form the conductive members 11 and 14 on the lead wire 13 of the power supply line 12 and on the electrode terminal 10 of the fluorescence tube 9 for backlight, a problem of increasing in the number of operations is caused. Moreover, since the contact member 6, the fluorescence tube 9 for backlight, and the power supply line 12 are not fixed, a problem of low reliability in positioning and holding is caused.
On the other hand, as shown in FIG. 57, in a case of a vertical (direct) type backlight in which a plurality of linear light sources 1 are arranged in parallel, HOT sides (high pressure sides) are placed in concentration on one side, and GND sides (low pressure sides) are placed in concentration on the other side. Thereby, a high luminance area H and a low luminance area G are generated due to a difference in the current densities of the HOT side and the GND side, causing a problem of luminance nonuniformity in the axial direction of the linear light sources 1. In other words, as shown in FIG. 58A, the linear light source 1 has a high potential on the HOT side, but the potential is attenuated to be low on the GND side, whereby a problem of luminance nonuniformity, in which the bright portion and the dark portion are generated, is caused in the display screen 18 of the liquid crystal display as shown in FIG. 58B so that the display definition is deteriorated.
Further, when a current supplied to each linear light source 1 has the same phase, flickers of adjacent linear light sources 1 are in the same frequency, causing a problem that flickers of the backlight as a whole are increased and electrical noises are caused to the liquid crystal panel.